Aircraft arrive at a landing site or airfield from many different directions and typically merge or converge into a single arrival stream before landing. This convergence ensures an orderly delivery to arrival and approach. A merging waypoint is used to define the merging point downstream of a terminal or runway at the airfield. Multiple aircraft routes are joined at the merging waypoint, and each aircraft follows the same path from the merging waypoint to the runway. However, prior the merging waypoint, the incoming aircraft need to be synchronized in time and spacing to allow for other aircraft to fit into the overall flow while maintaining a minimum separation between aircraft.
In general, a Continuous Descent Arrival (CDA) is an optimal arrival path or route for an aircraft towards a merging waypoint. In a CDA, the descent toward the merging waypoint is performed at near idle thrust values with little or no tactical intervention from Air Traffic Control (ATC). As such, a CDA results in optimum fuel, emissions and noise savings. However, with a CDA, it is difficult to systematically keep the aircraft flights separated in higher traffic density conditions because of inherent uncertainties associated with these operations. Therefore, ATC is employed to resolve conflicts between arriving aircraft and ensures that the aircraft remain separated at the merging waypoint.
The arrivals problem may be considered as a series of pairs of aircraft that converge to the same merging waypoint. In other words, solving an arrivals problem may be thought of as solving each one of these pair wise conflicts. Depending on the aircraft positions and speeds, the aircraft may stay separated without the need for ATC intervention. However, the ATC maintains control and takes actions to ensure appropriate separation.
Pair wise conflicts are solved by the ATC using one of the following two known techniques, both of which operate by attempting to cause a time delay in one of the aircraft in order to build sufficient spacing between the aircraft pair at the merging waypoint. The first technique is a radar vectoring or path stretching concept. Prior to reaching the merging waypoint, one of the aircraft is given a new lateral path that is relatively longer than a nominal path, thereby causing a delay in one of the aircraft. The Eurocontrol Point Merge (PM) procedure, for example, is a known procedure that follows the radar vectoring or path stretching concept. The PM procedure causes one of the aircraft to hold altitude and extend its flight path to solve the conflict. In general, this procedure has a relatively high success rate. However, because of the longer flight path and the altitude holding requirements, the PM procedure results in higher fuel use.
The second technique is a speed control concept. Prior to reaching the merging waypoint, one of the aircraft is slowed down, thereby causing a delay in the aircraft. The Optimized Profile Descent (OPD) procedure, for example, is a known procedure that utilizes the speed control concept to avoid conflicts while keeping the aircraft on the nominal path. The OPD procedure enables aircraft to more easily to complete CDAs and, thus, perform more fuel efficient descents. However, speed control often does not provide sufficient latitude to solve all of the pair wise conflicts and, thus, ATC intervention is still needed.